The broad, long-term goal of the proposed project is to provide a rational basis for the treatment of ventricular fibrillation (VF), a major cause of sudden cardiac death. The onset of VF causes the heart to stop pumping blood, which leads to ischemia (the lack of the blood flow) in the body and in the heart itself. Prolonged ischemia alters the cardiac state in such a way that it makes most of the attempts of defibrillation and cardio-pulmonary resuscitation futile. Empirical evidence indicates that the structure of VF ECG waveform is predictive of survival. The nature of this connection remains unknown. However, the ECG waveform reflects the spatiotemporal organization of electrical wavelets which sustain VF. Therefore, understanding the mechanisms linking the spatiotemporal organization of VF to ischemia is a necessary step towards determining the factors of survival, especially after prolonged times of VF/ischemia. Here we propose to use a combination of single cell, whole heart, and whole animal studies to establish a link between ischemia-specific alterations of cellular electrophysiology, spatiotemporal dynamics of fibrillatory waves, and the ECG waveform. The overall hypothesis is that the ischemia-induced changes in the balance between inward and outward currents, as well as in the coupling between the action potential and intracellular calcium cycling, lead to a dynamic instability in the action potential shape. This instability translates into a progressive disorganization of propagating wavelets and explains the progressive loss of periodicity in the ECG waveform. The specific aims are: 1. To analyze the mechanisms of dynamic instabilities of the action potential and Ca transient in isolated ventricular myocytes subjected to simulated conditions of ischemic VF. 2. To analyze mechanisms whereby dynamic instabilities of the AP and Ca transient cause conduction abnormalities during simulated and real ischemic VF in the whole heart. 3. To establish the link between the temporal regularity of the action potential, the dynamics of propagating wavelets and the ECG during natural VF evolution in-situ. Ventricular fibrillation (VF) is major cause of sudden cardiac death. The proposed research should explain physiological mechanisms linking the changes in the electrical activity during VF to the arrest of blood flow in the heart. It should also provide a scientific basis for the interpretation of VF ECG. This should help to optimize defibrillation and cardio-pulmonary resuscitation in the setting of out-of-hospital sudden cardiac arrest.